Email gpc115@gmail.com and you will get a reply from the person who wrote the app.
No. Every feature works without signing in — the budget, the forecast, money set aside for one-off events, rentals, debts and net worth all run on your phone. Signing in does one thing: it keeps a second phone in your household showing the same budget.
On the first phone, go to Settings → Sharing with your partner, sign in with your email, and start a household. You will be given a six-character join code. On the second phone, sign in and enter that code.
Codes arrive by email and expire after an hour. Check spam, then request a fresh one — an old code cannot be reused. If nothing arrives at all, email support and say which address you used.
Settings → Sharing with your partner → Delete account. That removes your sign-in and stops the two phones syncing. Your budget stays on your phone; to remove that as well, use Clear all budget data.
Only to spot events that usually cost money, so it can set money aside in advance. It is optional, it is read on your phone, and nothing from your calendar is ever uploaded. You can say no and the rest of the app is unaffected.
On your phone. If you turn on syncing, also in a database that only your household can read. See the privacy policy.